1. Field of the Invention
The present invention relates to a doping apparatus and a method of doping used for the manufacture of semiconductor integrated circuits and the like. More particularly, the present invention relates to an ion doping apparatus and a method of doping having a configuration preferable for processing substrates having large areas, wherein ion beams are radiated to a semiconductor material composed of amorphous components partly or entirely or to a substantially intrinsic polycrystalline semiconductor material to supply impurities to the semiconductor material.
2. Description of Related Art
Methods of forming p-type and n-type impurity regions in a semiconductor during the manufacture of semiconductor integrated circuits and the like are known in which ions of impurities that produce n and p conductivity types (n-type impurities and p-type impurities) are radiated and implanted by accelerating them by a high voltage. Especially, a method of separating mass and charge ratio of ions is referred to as "ion implantation" and are widely used for the manufacture of semiconductor integrated circuits.
Another method is known in which plasma including n-type and p-type impurities is produced and ions in the plasma are accelerated by a high voltage to be implanted in a semiconductor as an ion current. This method is referred to as "ion doping" or "plasma doping".
The structure of a doping apparatus utilizing ion doping is simpler than that of a doping apparatus utilizing ion implantation. For example, to implant boron as p-type impurities, plasma is produced in a gas of diborane (B.sub.2 H.sub.6) which is a boron compound by means of RF discharge or the like and a high voltage is applied to the plasma thereto to extract ions including boron which are in turn radiated into a semiconductor. Since gas-phase discharge is performed to produce plasma, the degree of vacuum inside the doping apparatus is relatively high.
Presently, an ion doping apparatus is frequently used to add impurities uniformly to a substrate having a relatively large area. This is because an ion beam to cover a large area can be relatively easily obtained in an ion doping apparatus which does not perform separation on a mass basis. On the other hand, for an ion implantation apparatus which must perform separation on a mass basis, it is difficult to increase the area of a beam while maintaining the uniformity of the ion. Therefore, an ion implantation apparatus is unsuitable for a substrate having a large area.
Recently, studies are active on the reduction of the temperature for semiconductor device processing. This is largely because of the fact that a necessity has arisen to form semiconductor devices on inexpensive insulated substrates made of glass and the like. Other reasons include needs associated with the trend toward microscopic devices and multi-layer devices.
Insulated substrates made of glass or the like have various advantages compared to silica substrates which have been used in processing at high temperatures in that they are easy to process, easy to form with a large surface area, inexpensive, and so on. However, as a matter of fact, the trend toward substrates having larger areas has also resulted in various difficulties to be technically overcome including a need for developing apparatuses having characteristics different from those suitable for conventional processes at high temperatures.
Ion implantation is disadvantageous for the manufacture of active matrix type liquid crystal displays and like wherein substrates having a large area must be processed, and ion doping is under research and development in an intention to cover such a disadvantage.
FIGS. 1 and 2 schematically illustrate a conventional ion doping apparatus. FIG. 1 schematically illustrates an ion source and an ion accelerator mainly. FIG. 2 illustrates the structure of the ion doping apparatus as a whole. The description will first proceed with reference to FIG. 1. Ions are generated in a plasma space 4.
Specifically, radio-frequency power is applied between an electrode 3 and a mesh electrode 6 by a radio-frequency power supply 1 and a matching box 2 to generate plasma in the plasma space 4 under a reduced pressure. Hydrogen or the like is introduced at the initial stage of plasma generation, and diborane and phosphine (PH.sub.3) which are doping gases are introduced after the plasma is stabilized.
The electrode 3 and the outer wall of the chamber (at the same potential as that of the mesh electrode 6) are insulated from each other by an insulator 5. An ion current is extracted from the plasma thus generated by an extraction electrode 10 and an extraction power supply 8. The ion current thus extracted is shaped by a suppressor grid 11 and a suppressor power supply 9 and thereafter accelerated into required energy by an acceleration electrode 12 and an acceleration power supply 7.
FIG. 2(A) will now be described. The ion doping apparatus is generally comprised of an ion source/accelerator 13, a doping chamber 15, a power supply device 14, a gas box 19, and an exhaust device 20. In FIG. 2, the ion source/accelerator as in FIG. 1 arranged horizontally. That is, in FIG. 2, the ion current flows to the right (downward in FIG. 1). The power supply device 14 mainly consists of power supplies used for generation and acceleration of ions and includes the radio-frequency power supply 1, matching box 2, acceleration power supply 7, extraction power supply 8, and suppressor power supply 9.
A substrate holder 17 is provided in the doping chamber 15, and a material 16 to be doped is placed thereon. In general, the substrate holder is designed such that it can be rotated about an axis in parallel with the ion current. The air in the ion source/accelerator 13 and the doping chamber 15 is exhausted by the exhaust device 20. The air in the ion source/accelerator 13 and the doping chamber 15 may be exhausted by separate exhaust devices.
A doping gas is delivered from the gas box 19 to the doping chamber 15 through a gas line 18. While a gas intake port is provided between the ion source/accelerator 13 and the material 16 to be doped in the apparatus shown in FIG. 2(A), it may be provided in the vicinity of the plasma space 4 of the ion source. The doping gas is generally used by diluting it with hydrogen or the like.
In the conventional ion doping apparatus, the area of a substrate (material to be doped) has been equal to or smaller than the sectional area of the plasma space 4 in the ion source 13. This is a requirement to be satisfied to achieve uniform doping. FIG. 2(B) illustrates a section which is perpendicular to the ion current. Specifically, the ion source/accelerator 13 has a size represented by L.sub.1 and L.sub.2, and the doping chamber 15 and a material 17 to be doped are sized such that they can be contained therein. The dimensions L.sub.1 and L.sub.2 are about the same.
Therefore, the size of the plasma space 4 must be increased with the size of the substrate. Further, plasma must have two-dimensional uniformity. However, it is difficult to increase the size of the plasma space infinitely. The reason is that this makes the generation of plasma nonuniform. This is primarily attributable to the fact that the mean free path of molecules becomes sufficiently smaller than the section of the plasma space. It is therefore difficult to make the length of one side of the plasma space equal to or greater than 0.6 m.